Present commercial color electrophotographic apparatus forms a series of electrostatic images on an image member, either a drum or a web, and tones the images with different colored toners. The images are then transferred in registration to a receiving sheet at a transfer station to produce a multicolor image.
Conventionally, the image member has been passed past a series of toning stations. The toning station having the color toner to be applied to a particular image is articulated into position in toning relation to that image and the image is toned. Other stations are positioned out of toning relation with that particular image and moved into such toning relation when the image destined to receive that station's particular color passes by. This approach requires articulation of each toning station which is expensive in both power and complexity of equipment. The station itself has to be moved into a position in which it is seated accurately with respect to the electrostatic image each time it moves, all of which can be done, but only with relatively expensive equipment.
U.S. Pat. No. 4,690,096, Hacknauer et al, issued Sept. 1, 1987, and U.S. Pat. No. 4,699,495, Hilbert, issued Oct. 13, 1987, are representative of a number of prior art references that suggest a gating mechanism between a developer delivery portion of a magnetic brush and a developer applicating portion of a magnetic brush to control the flow of such developer and allow the station to be turned on and off by positioning of the gating mechanism. These structures allow the toning stations to be permanently located in a toning position with respect to the image member, which in turn assures critical positioning. In general, the gating structures, although somewhat complex, are less expensive and complicated than articulation devices.
U.S. Pat. Nos. 4,473,029, Fritz et al, issued Sept. 25, 1984; 4,531,832, Kroll et al, issued July 30, 1985; and 4,546,060, Miskinis et al, issued Oct. 8, 1985; disclose a method of toning in which hard magnetic carrier particles and insulative toner particles make up a body of developer which is transported by a rapidly rotating magnetic core around a sleeve and into development position with respect to an electrostatic image. This sleeve can be stationary or rotated in either direction, but the developer flows generally in a direction opposite that of the rotating core. This structure has substantial advantages over traditional magnetic brush structure including extremely high quality development of fine lines and solid areas. The high coercivity of the carrier particles causes them to flip in response to pole transitions created by the rapidly rotating magnetic core which flipping is in a direction that carries them around the sleeve in a direction opposite to that of the moving core. In the toning area, the particles do not brush roughly over the electrostatic image, but instead are continually flipping at such a rapid rate that toning is effected without relative movement between the developer and the image. U.S. Pat. No. 4,922,302, Hill et al, issued May 2, 1990 shows an example of a toning station using this basic approach, which station has been adopted commercially.